Oil-based systems and methods that cool heat generating sources, such as electric transformers used in the field of railways to cool, for example, transformers feeding motors of electric locomotives, electric trains, or the like, are known in the art.
In U.S. Pat. No. 854,277, a system is disclosed for cooling an electric transformer, in particular a transformer used in the field of railways. This cooling system avoids the use of pumps, blowers and other structural parts requiring maintenance by submerging the transformer in a bath of cooling oil having a volume sufficient to absorb the heat generated by the transformer without overheating the transformer.
In U.S. Pat. No. 1,504,625, a system is disclosed for cooling an electric transformer, in particular an electric transformer used for feeding motors of electric locomotives, electric trains, or the like. This cooling system provides air as the cooling fluid, and the disclosed transformer is constructed to provide a flow of cooling air that is better distributed through the use of transformer coils, increasing cooling efficiency.
US 2006/0017537 generally discloses a cooling system of the type described hereinbefore, in which a cooling oil flows within a feeding circuit between two heat exchangers. A first exchanger absorbs the heat from the transformer and transmits it to the cooling oil, and a second exchanger absorbs the heat from the cooling oil and transmits it to the external environment, lowering the temperature of the cooling oil that is again fed to the first heat exchanger.
A drawback of oil cooling systems is that the oil flowing into the cooling circuit is monitored for safety reasons. This is accomplished by using flow meters or differential pressure sensors.
Flow meters are generally composed of a mechanical member, such as a paddle or the like whose deflection is correlated to flow velocity. When there is no flow or when the fluid flow is too slow, i.e. it is below a minimum threshold velocity, the paddle is not deflected and the flow meter is not able to detect the presence of the fluid flow.
This effect occurs when the fluid flow is too slow, but also and particularly under relatively low temperatures when the fluid and particularly the oil are subjected to an increase in viscosity.
Therefore, under such conditions conventional flow meters are not able to indicate the presence of a fluid flow or such indication is not reliable.
Differential pressure sensors are an alternative for determining the presence of a fluid flow into the circuit of a cooling system. The pressure drop occurring between the inlet and outlet of one of the heat exchangers is detected by means of such sensors. Differential pressure sensors do not have the drawbacks of flow meters when the flow is very slow or when the cooling fluid, particularly the cooling oil, has a greater viscosity.
Differential pressure sensors are not very reliable, so they must be redundant, i.e. the circuit has to be provided with more than one differential pressure sensor, particularly for guaranteeing the safety levels required in the railway field. Such unreliability leads to a more burdensome construction and, above all, higher costs of the cooling system.
In cooling systems where the cooling fluid is oil, the drawback related to viscosity increases and, accordingly, the poor reliability of the signals about cooling oil flow detected by the flow meters becomes noticeable at temperatures equal to or lower than 10° C., and becomes more and more relevant as the temperature decreases. Therefore, the poor reliability of the flow meters is not a minor drawback that occurs under extreme environmental conditions, but is a drawback having deleterious effects at the room temperatures that are normal and usual in most parts of the world.